Sensitometry



Oct. 17, 1933. s. s A. WATKINS 1,931,046

SENSITOMETRY Filed Aug. 20, 1931 FIG. Z.

1 El E3 IN l ENTOR 5.5.A. MIT/(INS AH drmt /way Patented Oct. 17, 1933UNITED STATES PATENT OFFICE,

SENSITOMETRY Application August 20, 1931, Serial N0.

and in Great Britain March 9, 1931 V 3 Claims. (01. 179-1003) Thisinvention relates to the recording of modulated waves on a photographicfilm and particularly to the recording of sound on a photographic filmby the variable area method.

The object oi the invention is a means for readily obtaining asensitometer record in the width of the film allotted to the soundtrack.

A. feature of the invention is the provision of switching meanswherebypredetermined values of current may be applied to the recordingdevice.

In the variable area method of recording modulated waves, a small beamof radiant energy of constant intensity is directed on a sensitive filmand affects a portion of the Width of the record track. A recordingdevice actuated by modulated waves causes the beam to afiect an areacovering more or less of the width of the Th control of the beam ofradiant energy may be affected by reflectingthe beam from a mirrorassociated with the recording device to the film. The mirror is vibratedby the modulated waves and causes the area illuminated by the beam to bedisplaced laterally of the film. Or, the width of the beam may bedefined by physical means, such as a shutter in the recording device,and the movement of this shutter under the control of the modulatedwaves beam laterally or the film.

In order to secure data for the proper control the photographicprocesses, the density of the exposed and unexposed portions of thesound track must be measured. The density of the sound track mayconveniently be measured by known types of densitometers, such as apolarization photometer. The field of a densitometer of this type isusually approximately circular and cannot conveniently be reduced to avery small area. In the known systems of recording in common use, thesound track is about inch wide, and when the beam is located in thecenter of the track, a strip of film inch wide is exposed and a similarstrip is unexposed.

Owing to the small width of the strip, and the large field of thedensitometer, the density cannot conveniently be measured.

In accordance with the present invention, switching means are providedto control a biasing current which is applied to the recording device.In the normal position of the switching means, a normal bias is appliedto the recording device and the recording beam causes an eX- posure ofone-half of the Width of the record track. In a second position of theswitching will cause a variation in the width of the.

means the bias is changed so that the beam is completely removed fromthe record track and the full width of the track is unexposed. In athird position of the switching means, the bias is changed so that thebeam covers the record track and the full width of the track is exposed.Densitometer readings may then conveniently be made on the sections ofthe record track in which the full width is respectively fully exposedor unexposed.

Fig. l discloses in schematic iorma recording system embodying apreferred form of the invention.

Fig. 2 discloses a section of film produced by the invention.

In Fig. 1 sound waves may be detected by the microphone 1 and areamplified in the known manner in an amplifier or amplifiers. The outputof the amplifier applied to the primary winding of a suitabletransformer 2. The output of transformer 2 is supplied through a largecapacitor 3 to the movable strings 4, 5 of an oscillograph orgalvanometer. The strings 4, 5 are partially immersed in the steadymagnetic field of the magnet 6. A small mirror 7 is afiixed to thestrings 4, 5. The reaction of the magnetic field due to an electriccurrent flowing in the strings 4 and 5 with the magnetic field due tothe magnet 6 will cause the mirror '7 to be vibrated.

Light from a suitable source 8 illuminates a small aperture in an opaqueplate 9. This aperture preferably has the shape of a small, longrectangle. An image of the aperture is formed by the lens 10 on themirror '7. The light falling on the mirror 7 is reflected as a smallrectangular beam which partially illuminates a small aperture in theplate 11. The light passing through the aperture in the plate 11 fallson a photographic film 12 which is moved past the aperture by thesprockets 13, 14 which are rotated at constant speed in a known mannerfrom a source of power, not shown. While, for convenience, a mirrorgalvanometer has been disclosed and described, it will be apparent tothose skilled in the art that many other types of light valves may beused and that the invention is not in any way limited by thegalvanometer shown.

Current from a battery 15 flows through the springs 16, 17, 18 and. 19of the key 28 and variable resistor 21 to the strings 4 and 5 of thegalvanometer, thence through inductor 22 to battery 15. The capacitor 3prevents this current from flowing in the secondary Winding of thetransformer 2. This constant current flowing through the strings of thegalvanometer causes the mirror '7 to be deflected, The result of biasingthe mirror '7 as above described is illustrated in Fig. 2 at the points23, 24 and 25 in the sound track on the film 26. When modu- 5, throughreactor 22 to battery 15.

lated currents are superimposed on this normal value of the biasingcurrent, a conventional sound record is produced, as illustrated at 27in Fig. 2. The reactor 22 prevents the modulated currents from beingdiverted through the biasing circuit.

To obtain a sensitometer record, before modulated currents are appliedto the strings, the key 28 is operated to the right, opening theconnection between the springs 16 and 17 and removing the biasingcurrent from the strings 4 and 5 of the galvanometer. The galvanometeris mechanically adjusted so that when no current flows in the strings 4and 5, the mirror '7 is deflected and the full width of the record trackis unexposed, as illustrated at 29 in Fig. 2.

After a record of convenient length of the unexposed track has beenmade, the key 28 is restored to normal, closing the connection betweensprings 16 and 17 and causing the recording device to deflect the beamof light exposing one-half of the record track as illustrated at 24 inFig. 2. The key 28 is then moved to the left opening the connectionbetween springs 13 and 19 and closing a connection between springs 19and. 20. Current flows from battery 15, through springs 16 and 1'7,battery 30, springs 20 and 19, adjustable resistor 21, strings 4 and Ifthe batteries 15 and 31 are of equal voltage, the mirror 7 will bedeflected so that the full width of the record track is exposed asillustrated at 31 in Fig. 2. A record of the fully exposed track '0!convenient length is made and the key 28 may then be restored to normal.The circuit is then in condition to record modulated currents.

What is claimed is:

1. In a film sound recording system of the variable area type, themethod of making a sensitometer record which comprises mechanicallyadjusting the recording device so that a portion of the record track issubstantially unexposed, applying a constant electric current to therecording device, manually adjusting said current so that anotherportion of the record track is exposed for substantially one-half of itswidth, and manually applying a further voltage such that said current issubstantially doubled so that a further portion of said record track isexposed for substantially its full width.

2. In an apparatus for producing a sensitometer record in combination,means for producing a small beam of light, means for moving a sensitivefilm at constant speed past a small aperture, means for deflecting saidbeam to just fail to illuminate said aperture, means for manuallyapplying an electric current to said defleeting means to cause said beamto illuminate substantially one-half of said aperture, and switchingmeans for manually applying a further electric current to saiddeflecting means to cause said beam to illuminate substantially thewhole of said aperture.

3. In an apparatus for producing a sensitometer record, in combination,means for producing a small beam of light, means for moving a sensitivefilm at constant speed past a small aperture, a recording deviceadjusted to deflect said beam to just fail to illuminate said aperture,and switching means for impressing at will two predetermined values ofelectric current on said recording device to cause said device todefiect said beam and illuminate said aperture to two predetermineddegrees.

STANLEY S. A. WATKINS.

